By THOMAS ERDBRINK and ALAN COWELL

September 17, 2013

TEHRAN — Iran’s supreme leader seemed to put his authority behind Iran’s moderate new president on Tuesday, calling for “heroic leniency” in navigating the country’s diplomatic dispute with the West.

The president, Hassan Rouhani, was elected in June on a moderate platform of ending the nuclear standoff with the West and increasing personal freedoms. In a speech to the Revolutionary Guards, considered stalwarts of the conservative wing of the government, the supreme leader, Ayatollah Ali Khamenei, said he was “not opposed to proper moves in diplomacy.”

Enlarging on that theme, he said, “I agree with what I called ‘heroic leniency’ years ago, because such an approach is very good and necessary in certain situations, as long as we stick to our main principles.”

In what may be a further signal that Mr. Rouhani’s victory in the June election has created a chance for intensified diplomacy, the country’s Foreign Ministry confirmed on Tuesday that he had exchanged letters with President Obama.

But asked about the tone of Mr. Obama’s letter — something the Iranians are extremely sensitive about — Marzieh Afkham, the ministry spokeswoman, said Iran expected improvement in the way Washington talked to Iran.

Iran’s supreme leader, Ayatollah Ali Khamenei.

Khamenei Official Website, via European Pressphoto Agency

“Unfortunately, the U.S. administration is still adopting the language of threat while dealing with Iran,” Ms. Afkham said at a weekly news conference. “We have announced that this needs to change into the language of respect.”

The United States and Iran have had no diplomatic relations since Washington ended ties after the seizure of 52 diplomatic personnel in 1979 after the Islamic revolution. Since his election, Mr. Rouhani has said he is interested in improving relations with the rest of the world, including the United States.

Ms. Afkham said Mr. Obama had initiated the exchange of letters by congratulating Mr. Rouhani on his election victory. Mr. Rouhani “expressed thanks for the congratulations” and wrote about various issues, she said, without elaborating on what they were.

Iran and the United States are at loggerheads over Tehran’s disputed nuclear program, and they are on opposing sides in Syria’s civil war. Iran is the most important regional ally of President Bashar al-Assad, while the United States supports the rebels seeking his ouster.

Ms. Afkham said the letters had been sent through the Swiss Embassy in Tehran, which has a section looking after American interests. In a television interview on Sunday, Mr. Obama also confirmed the exchange of letters.

Word of the correspondence emerged as both leaders prepared to address the United Nations General Assembly in New York next week. Ms. Afkham said no meetings were scheduled between Iranian and United States officials. Mr. Rouhani will also not meet with the British foreign secretary, William Hague, she said, dismissing a message on a Twitter account affiliated with Mr. Rouhani asking for such a meeting.

President Hassan Rouhani.

Iranian Presidential Website, via European Pressphoto Agency

The exchange of letters also coincided with a decision by the Obama administration last week to ease longstanding restraints on humanitarian and good-will activities between Iran and the United States, including athletic exchanges. It was at least the second relaxation of Iranian sanctions this year by the American government.

On another diplomatic front, Prime Minister Benjamin Netanyahu of Israel said Tuesday that he would focus on halting Iran’s nuclear program in a meeting with Mr. Obama in Washington at the end of the month and in his annual speech before the United Nations General Assembly, using the Syria situation to increase pressure for a “credible military threat.”

Repeating a four-step formula he has been urging for months, Mr. Netanyahu told his cabinet that Iran must stop enriching uranium, remove enriched uranium from the country, close its nuclear plant near Qum and stop what he called “the plutonium track.”

“Until all four of these measures are achieved, the pressure on Iran must be increased and not relaxed, and certainly not eased,” the prime minister said in a statement released by his office.

Negotiations intended to resolve the nuclear dispute have been deadlocked since before Mr. Rouhani’s election, which has been depicted as a potential opportunity to break the stalemate.

Iran, arguing that its program is for peaceful purposes, insists on its right to enrich uranium. But Western powers fear that Tehran’s scientists are seeking the capability to build nuclear weapons, a prospect that alarms both the United States and Israel.

Mr. Rouhani has not indicated whether Iran will shift positions, but he has replaced the most senior officials in the program. Last month, in their first report since Mr. Rouhani took office, nuclear inspectors from the United Nations said Iran had slowed its accumulation of uranium that could be quickly converted to bomb fuel.

Correction: September 17, 2013

An earlier version of this article misstated the timing of a report on Iran’s nuclear program by United Nations inspectors. It was released last month, not last week.